1. Field of the Invention
The present invention relates generally to an active suspension system for an automotive vehicle, which is controlled in an active manner for absorbing road shock and suppressing vehicular body attitude change in order to achieve both riding comfort and driving stability. More specifically, the invention relates to an actively controlled suspension system which has variable control characteristics depending upon road friction so that driving stability on a low friction road can be enhanced.
2. Description of the Background Art
European Patent First Publication No. 0 283 004 discloses an actively controlled suspension system including an anti-roll control loop which is responsive to lateral acceleration to be exerted on a vehicle to adjust suspension characteristics. The control loop employed suspension system is variable of response characteristics to vehicular rolling depending upon the vehicle speed for varying anti-rolling suspension control characteristics. In a range of the vehicle speed in which the greater loop gain of the control loop causes significant and unacceptable levels of self-induced lateral vibration, a gain is varied to be the smaller value than that in other vehicle speed ranges. Namely, since the greater gain may cause self-induced lateral vibration in a substantially low vehicle range, the gain main be adjusted to a smaller value while vehicle speed is lower than a given value. Also, European Patent First Publication No. 0 285 153 discloses an actively controlled automotive suspension system employing a factor representative of a self-induced attitude change creative factor as a correction factor in an attitude regulating suspension control. Suspension characteristics are controlled on the basis of a rolling and/or pitching energy representing parameter. The correction factor representative of self-induced attitude change is derived on the basis of the vehicular attitude change velocity for canceling the self-induced rolling and/or pitching. By canceling the self-induced rolling and/or pitching creative component from the rolling and/or pitching acceleration factor, higher rolling and/or pitching response characteristics can be provided so as to obtain satisfactorily high driving stability and riding comfort.
On the other hand, Japanese Patent First (unexamined) Publication (Tokkai) Showa 62-295714, which is assigned to the common assignee to the present invention, discloses one typical example of a prior proposed actively controlled suspension system for performing anti-rolling and/or anti-pitching suspension controls. The disclosed system employs a lateral acceleration sensor and a longitudinal acceleration sensor for monitoring vehicular attitude change magnitude indicative parameters for performing attitude change suppressive control.
In the prior proposed anti-rolling suspension control systems, magnitude of adjusting stiffness of suspension characteristics of the suspension system is determined depending upon a lateral acceleration exerted on the vehicular body with a uniformly set control characteristic. Therefore, irrespective of the road surface condition, i.e. dry or wet, high friction or low friction, the control magnitude with respect to the magnitude of lateral acceleration is held unchanged. Therefore, distribution of rolling moment at the front and rear suspension systems is maintained constant to cause degradation of cornering ability and/or steering characteristics on a low friction road.